Synthesis and processing of the 35S rRNA precursor in Tetrahymena pyriformis in vivo and in vitro is being investigated. A physical map of the rRNA gene has been constructed using several restriction endonucleases. The locations of the 5' and 3' termini of the 35S rRNA on the physical map have been determined; Hind III fragments containing most of the transcribed regions of the rRNA gene have been cloned in E coli K12 using the vector pBR 322. We are currently searching for a clone containing the DNA fragment which encodes the 5' terminus of the 35S rRNA. We are determining the DNA sequence in the region of the 3' end of the 35S rRNA. We are also sequencing the 3' ends of the 35S rRNA and 26S rRNA. As soon as the DNA fragment which encodes the 5' end of the 35S rRNA is cloned, we will determine the base sequence of the DNA and the 5' end of the 35S rRNA.